Numerous publications have demonstrated that beer and the bitter acids found in beer are beneficial to health. As such, light-stable reduced hops bitter acid extracts have been used to help treat or prevent various conditions such as diabetes, cancer, inflammation, and obesity. The female cones of Humulus lupulus L. (hops) produce a mixture of humulones and lupulones known as alpha- and beta-acids, respectively. The group of alpha-acids used as the main bittering agent in conventional commercial brewing methods, known as the reduced iso-alpha-acids, are thought to be responsible for many of the beneficial effects of beer.
There are several categories of reduced iso-alpha-acids, each existing naturally as a mixture of congeners and stereoisomers, and each having independent biological activity. The categorization depends on the location and degree of saturation. The class of compounds resulting from the reduction of only the C6 carbonyl to a hydroxyl is collectively referred to as the rho iso-alpha-acids (RIAAs). Compounds where only both isoprenyl moieties are saturated are referred to as tetrahydro iso-alpha-acids (THIAAs). In addition to the existence of cis and trans diastereomers, there are a variety of congeners within each of the three classes as a result of the incorporation of various short-chain fatty acids into the biosynthetic pathway of the corresponding phloroglucinols (Wang, 2008). The phloroglucinols are common precursors to the alpha-acids that are precursors to iso-alpha-acids, which are in turn precursors to the reduced iso-alpha-acids.
Among other biological activities, mixtures of reduced iso-alpha-acids have been shown to inhibit the activity of several enzymes (e.g., spleen tyrosine kinase, Bruton's tyrosine kinase, phosphatidylinositol 3-kinase), inhibit osteoclastogenesis, reduced arthritis index, decreased bone, joint and cartilage degradation, reduce IL-6 levels, inhibit prostaglandin E2 production, inhibit inducible cyclooxygenase-2 protein expression, and reduce NFκB translocation and abundance. However, the heterogeneity of the mixtures prevents a clear understanding of the relationships between each of the congeners and stereoisomers present with respect to their individual and relative biological activities. As such, synthesis of enantiomerically pure reduced iso-alpha-acids is desired to ensure accurate biological analysis and structure-function relationships.
Most commercial routes of synthesis are dependent upon natural extracts of the alpha- and beta-acids found in hops for a starting material, and result in products of varying purity. Notwithstanding the purity of products produced using natural extracts, these routes of synthesis also present challenges with respect to the availability and sourcing of starting materials due to time, cost, and agricultural variability. As such, there is a need for methods that can produce enantiomerically pure reduced iso-alpha-acids for use in pharmaceutical compositions and treatments, which also address the issues discussed above related to the starting materials.
A total synthesis of the iso-alpha-acids from phloroglucinol that was previously described in the literature suffered from low yields and produced only racemic material (Ting, 2009). The development of a commercially viable, robust total synthesis of the iso-α-acids would allow labeling and chemical derivatization of the hops bitter acids to enable further investigation of biological structure-activity relationships (SAR) and would support pharmaceutical development. However, there are three steps within the total synthesis of the iso-α-acids from phloroglucinol where a large percentage of an undesired product can result, including prenylation, oxidation (undesired enantiomer) and isomerization (undesired diastereomer). Likewise, the synthesis of iso-alpha-acids from natural hops extracts also produces undesired product, which contributes to low yields. Therefore, there is a need to optimize the methods of synthesis from natural hop extracts and precursors, such as phloroglucinols, to prepare enantiomerically pure or substantially enantiomerically pure reduced iso-alpha-acid compounds and their derivatives for use in pharmaceutical compositions and treatments.